Existing methods of welding filter bag bottoms are generally slow and not readily adaptable to an automated assembly line or result in relatively poor quality welds, which is undesirable. One method of welding bag bottoms, the hot air nozzle method, uses two pre-cut bag halves that are passed along a hot air welder. An operator inserts the hot air nozzle into the bag at a starting point; activates the hot air nozzle machine, softening each bag bottom half; places the semi-molten surfaces between a pair of driven nip rolls to seal the halves together; stops the machine once one side of the bag is welded; flips the bag over; and repeats the process for the other side of the bag. The bag has to be removed from the nozzle at a precise point, so that the nozzle will not damage the end of the bag. This process is slow, as it is entirely dependent upon the speed of the operator working the hot air nozzle machine, and results in poor quality welds having improperly aligned bag halves, loose fibers along the bottom edges of the bag due to the required pre-cutting, or welds of varying margins.
A second method for welding bag bottoms is the hot plate-type method. In this method, a heated tong-type plate, shaped to match the shape of the bottom of the bag, is inserted between the two layers of the bag bottom. The plate is withdrawn after the inner surfaces of the two halves have been slightly softened, and the halves are then pressed together to bond. This method is difficult to control, since the width of the bag must precisely match the width of the heated plate, which does not permit even slight variation of the bag width. Using an under-sized (i.e., narrow) bag will create damaged molten edges, while using an over-sized (i.e., wide) bag will create channels of unwelded sections along the bag bottom. Since controlling the bag width to close tolerances is difficult, this method is impractical to use.
A third method of welding bag bottoms is the ultrasonic plunge welding method. A major disadvantage to this method is that each side of the bag has to be welded separately, since ultrasonic welding is not capable of welding components longer than twelve inches. The quality of the weld depends upon uniformity of the thickness and density of a given material, which varies widely with filtration media felts. Additionally, ultrasonic welding results in welds of a lesser strength than other methods, mainly due to the fact that energy is transferred from the outside of the bag to the interface of the two layers of the bag bottom, which requires excessive melting of the outer surface in order to achieve a strong weld at the interface of the two layers. Ultrasonic welding also has a tendency to cause embrittlement of the weld due to the high level of energy required in the welding process.
In order to be utilized in an assembly line process, the method used to weld filter bag bottoms must be fast and result in high-quality welds. The methods of the prior art are generally slow and result in poor quality welds because the various factors contributing to each weld (for instance, the temperature achieved during the weld, the weld margin, and the force applied to bond the softened surfaces together) cannot be precisely controlled. There is therefore a need in the art for a method to weld filter bag bottoms that allows precise control over the quality of the weld and that is fast enough to be adapted for use in an assembly line environment.